1. Field of the Invention
The present invention generally relates to methods and systems for processing carbon nanotubes, and more specifically, to isolating single-walled carbon nanotubes from double and multi-walled carbon nanotubes.
2. Background Art
Single-Walled Carbon Nanotubes (SWCNTs), commonly known as “buckytubes,” have attracted enormous interest due to their excellent electrical, optical and mechanical properties. SWCNTs are hollow tubular fullerene molecules consisting essentially of sp2-hybridized carbon atoms typically arranged in hexagons. Single-wall carbon nanotubes typically have diameters in the range of about 0.5 nanometers (nm) and about 3.5 nm, and lengths usually greater than about 50 nm.
A key technological challenge is growing high-quality SWCNTs in large quantities. Although Chemical Vapor Deposition (CVD) provides the highest quality SWCNTs, large quantity forest growth is hampered by the presence of Double and Multi-Walled CNTs. The ratio of Single-Walled to Multi-Walled CNTs can vary greatly from one process to another and is difficult to control.
Several methods of synthesizing fullerenes have developed from the condensation of vaporized carbon at high temperature. Fullerenes, such as C60 and C70, may be prepared by carbon arc methods using vaporized carbon at high temperature. Carbon nanotubes have also been produced as one of the deposits on the cathode in carbon arc processes.
Single-wall carbon nanotubes have been made in a DC arc discharge apparatus by simultaneously evaporating carbon and a small percentage of Group VIIIb transition metal from the anode of the arc discharge apparatus. These techniques allow production of only a low yield of carbon nanotubes, and the population of carbon nanotubes exhibits significant variations in structure and size.
Another method of producing single-wall carbon nanotubes involves laser vaporization of a graphite substrate doped with transition metal atoms (such as nickel, cobalt, or a mixture thereof) to produce single-wall carbon nanotubes. The single-wall carbon nanotubes produced by this method tend to be formed in clusters, termed “ropes,” of about 10 to about 1000 single-wall carbon nanotubes in parallel alignment, held by van der Waals forces in a closely packed triangular lattice. Nanotubes produced by this method vary in structure, although certain structures may predominate. Although the laser vaporization process produce can produce improved yields of single-wall carbon nanotubes, the product is still heterogeneous, and the nanotubes tend to be too tangled for many potential uses of these materials.
Another way to synthesize carbon nanotubes is by catalytic decomposition of a carbon-containing gas by nanometer-scale metal particles supported on a substrate. The carbon feedstock molecules dissociate on the metal particle surface and the resulting carbon atoms combine to form nanotubes. The method typically produces imperfect multi-walled carbon nanotubes. One example of this method involves the disproportionation of CO to form single-wall carbon nanotubes and CO2 catalyzed by transition metal catalyst particles comprising Mo, Fe, Ni, Co, or mixtures thereof residing on a support, such as alumina. Although the method can use inexpensive feedstocks and moderate temperatures, the yield of single-wall carbon nanotubes can be low, with large amounts of other forms of carbon, such as amorphous carbon and multi-wall carbon nanotubes present in the product. The method often results in tangled carbon nanotubes and also requires the removal of the support material for many applications.
All-gas phase processes can be used to form single-wall carbon nanotubes. In one example of an all gas-phase process, single-wall carbon nanotubes are synthesized using benzene as the carbon-containing feedstock and ferrocene as the transition metal catalyst precursor. By controlling the partial pressures of benzene and ferrocene and by adding thiophene as a catalyst promoter, single-wall carbon nanotubes can be produced. However, this method suffers from simultaneous production of multi-wall carbon nanotubes, amorphous carbon, and other products of hydrocarbon pyrolysis under the high temperature conditions necessary to produce high quality single-wall carbon nanotubes.